Printhead color density correction in printing systems

ABSTRACT

A method for color density correction in a printing system that includes a linehead, with one or more printheads, that jets ink onto a moving print media and an integrated imaging system that captures images of content printed on the moving print media is provided. One or more pixel data values and a measured density value trace for a printed test block are produced by scanning the test block and averaging pixel data in a print media transport direction. A color and a density of the ink in the printed test block are determined using the pixel data values. The measured density value trace is compared with a respective reference density value. It is determined whether there is a difference between the measured density value trace and a reference density value is determined. If there is a difference, ink laydown for the printhead is adjusted based on the difference.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is related to U.S. patent application Ser. No.13/747,573, entitled “PRINTHEAD COLOR DENSITY CORRECTION IN PRINTINGSYSTEMS”, filed concurrently herewith. This patent application isrelated to U.S. patent application Ser. Nos. 13/332,415 and 13/332,417,both filed on Dec. 21, 2011. This patent application is related to U.S.patent application Ser. Nos. 13/537,240 and 13/537,247, both filed onJun. 29, 2012.

TECHNICAL FIELD

The present invention generally relates to printing systems and moreparticularly to methods for printhead color density correction inprinting systems.

BACKGROUND

In commercial inkjet printing systems, the lineheads typically includemultiple printheads that jet ink or another substance onto a printmedia, such as paper. Each printhead can include a nozzle plate havingprecisely sized and spaced nozzles. The diameter of each nozzle canrange from five to twenty micrometers. Because multiple nozzle platesare used in many printing systems, the number of nozzles that arefabricated for each linehead can range between 12,000 to 30,000 nozzles.

It can be challenging to fabricate such small nozzles uniformly andconsistently, along with the other linehead components associated withink ejection. Failure to precisely fabricate the components within andbetween nozzle plates can lead to non-uniformities in the contentprinted by the printing system. The resulting variations in ink lay downcharacteristics can lead to unpredictable variations in dark and lightdensity regions. The dark and light density regions continue untilcorrected, but the necessary corrections may not occur for hundreds orthousands of feet of print media. The non-uniformities in the printedcontent can result in waste when the printed content is not usable.Additionally, the wasted print media causes a print job to be morecostly and time consuming.

SUMMARY

In one aspect of the invention, a printing system includes one or morelineheads for jetting ink or liquid onto a moving print media and anintegrated imaging system that captures one or more images of at leastone test block printed on the moving print media. The integrated imagingsystem includes a housing, an opening in the housing for receiving lightreflected from the print media, a folded optical assembly in the housingthat receives the reflected light and transmits the light apredetermined distance, and one or more image sensors within the housingthat each receive the light and capture one or more images of theprinted test block or blocks on the moving print media. The image sensoror sensors each include a color filter array having a known captureresponse. The color filter array or arrays can be complementary to theink colors. The imaging system is connected to an image processingdevice. The image processing device receives pixel data from the one ormore image sensors and is configured to determine a color of the ink anda density of the at least one printed test block.

In another aspect of the invention, a method for color densitycorrection in a printing system is provided. A printing system includesa linehead that jets ink onto a moving print media and an integratedimaging system that captures images of content printed on the movingprint media. The linehead includes one or more printheads and theintegrated imaging system includes one or more image sensors havingcolor filter arrays with known capture responses. The method includesproducing one or more pixel data values and a measured density valuetrace for a printed test block by scanning the test block and averagingpixel data in a print media transport direction and determining a colorand a density of the ink in the printed test block using the pixel datavalues. The measured density value trace is compared with a respectivereference density value. A determination is made as to whether or notthere is a difference between the measured density value trace and areference density value. If there is a difference, adjusting ink laydownfor the printhead based on the difference.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are better understood with reference to thefollowing drawings. The elements of the drawings are not necessarily toscale relative to each other.

FIG. 1 illustrates one example of an inkjet printing system that printson a continuous web of print media;

FIG. 2 depicts a portion of printing system 100 in more detail;

FIG. 3 illustrates a side of the support structure 204 that is oppositethe print media 112 in an embodiment in accordance with the invention;

FIG. 4 depicts a portion of a printing system in an embodiment inaccordance with the invention;

FIG. 5 is a cross-sectional view along line 5-5 in FIG. 4 in anembodiment in accordance with the invention;

FIG. 6 is a cross-sectional view along line 6-6 in FIG. 4 in anembodiment in accordance with the invention;

FIG. 7 is a flowchart of a method for printhead color density correctionin a printing system in an embodiment in accordance with the invention;

FIG. 8 depicts an example of a test block pattern in an embodiment inaccordance with the invention;

FIG. 9 illustrates an example of a spectral response plot in anembodiment in accordance with the invention;

FIG. 10 depicts the spectral response plot for the magenta ink shown inFIG. 9;

FIG. 11 illustrates the spectral response plot for the yellow ink shownin FIG. 9; and

FIG. 12 depicts one example of density value traces for the printheadsshown in FIG. 3 in an embodiment in accordance with the invention;

DETAILED DESCRIPTION

Throughout the specification and claims, the following terms take themeanings explicitly associated herein, unless the context clearlydictates otherwise. The meaning of “a,” “an,” and “the” includes pluralreference, the meaning of “in” includes “in” and “on.” Additionally,directional terms such as “on”, “over”, “top”, “bottom”, “left”, “right”are used with reference to the orientation of the Figure(s) beingdescribed. Because components of embodiments of the present inventioncan be positioned in a number of different orientations, the directionalterminology is used for purposes of illustration only and is in no waylimiting.

The present description will be directed in particular to elementsforming part of, or cooperating more directly with, an apparatus inaccordance with the present invention. It is to be understood thatelements not specifically shown, labeled, or described can take variousforms well known to those skilled in the art. In the followingdescription and drawings, identical reference numerals have been used,where possible, to designate identical elements. It is to be understoodthat elements and components can be referred to in singular or pluralform, as appropriate, without limiting the scope of the invention.

The example embodiments of the present invention are illustratedschematically and not to scale for the sake of clarity. One of ordinaryskill in the art will be able to readily determine the specific size andinterconnections of the elements of the example embodiments of thepresent invention.

As described herein, the example embodiments of the present inventionprovide a printhead or printhead components typically used in inkjetprinting systems. However, many other applications are emerging whichuse inkjet printheads to emit liquids (other than inks) that need to befinely metered and deposited with high spatial precision. Such liquidsinclude inks, both water based and solvent based, that include one ormore dyes or pigments. These liquids also include various substratecoatings and treatments, various medicinal materials, and functionalmaterials useful for forming, for example, various circuitry componentsor structural components. As such, as described herein, the terms“liquid” and “ink” refer to any material that is ejected by theprinthead or printhead components described below.

Inkjet printing is commonly used for printing on paper. However, thereare numerous other materials in which inkjet is appropriate. Forexample, vinyl sheets, plastic sheets, textiles, paperboard, andcorrugated cardboard can comprise the print media. Additionally,although the term inkjet is often used to describe the printing process,the term jetting is also appropriate wherever ink or other liquids isapplied in a consistent, metered fashion, particularly if the desiredresult is a thin layer or coating.

Inkjet printing is a non-contact application of an ink to a print media.Typically, one of two types of ink jetting mechanisms are used and arecategorized by technology as either drop on demand ink jet (DOD) orcontinuous ink jet (CIJ). The first technology, “drop-on-demand” (DOD)ink jet printing, provides ink drops that impact upon a recordingsurface using a pressurization actuator, for example, a thermal,piezoelectric, or electrostatic actuator. One commonly practiceddrop-on-demand technology uses thermal actuation to eject ink drops froma nozzle. A heater, located at or near the nozzle, heats the inksufficiently to boil, forming a vapor bubble that creates enoughinternal pressure to eject an ink drop. This form of inkjet is commonlytermed “thermal ink jet (TIJ).”

The second technology commonly referred to as “continuous” ink jet (CIJ)printing, uses a pressurized ink source to produce a continuous liquidjet stream of ink by forcing ink, under pressure, through a nozzle. Thestream of ink is perturbed using a drop forming mechanism such that theliquid jet breaks up into drops of ink in a predictable manner. Onecontinuous printing technology uses thermal stimulation of the liquidjet with a heater to form drops that eventually become print drops andnon-print drops. Printing occurs by selectively deflecting one of theprint drops and the non-print drops and catching the non-print drops.Various approaches for selectively deflecting drops have been developedincluding electrostatic deflection, air deflection, and thermaldeflection.

Additionally, there are typically two types of print media used withinkjet printing systems. The first type is commonly referred to as acontinuous web while the second type is commonly referred to as a cutsheet(s). The continuous web of print media refers to a continuous stripof media, generally originating from a source roll. The continuous webof print media is moved relative to the inkjet printing systemcomponents via a web transport system, which typically include driverollers, web guide rollers, and web tension sensors. Cut sheets refer toindividual sheets of print media that are moved relative to the inkjetprinting system components via rollers and drive wheels or via aconveyor belt system that is routed through the inkjet printing system.

The invention described herein is applicable to both types of printingtechnologies. As such, the terms printhead and linehead, as used herein,are intended to be generic and not specific to either technology.Additionally, the invention described herein is applicable to both typesof print media. As such, the terms web and print media, as used herein,are intended to be generic and not as specific to either type of printmedia or the way in which the print media is moved through the printingsystem.

The terms “upstream” and “downstream” are terms of art referring torelative positions along the transport path of the print media; pointson the print media move along the transport path from upstream todownstream. In FIGS. 1-3 the media moves in the direction indicated bytransport direction arrow 114. Where they are used, terms such as“first”, “second”, and so on, do not necessarily denote any ordinal orpriority relation, but are simply used to more clearly distinguish oneelement from another.

Referring now to the schematic side view of FIG. 1, there is shown oneexample of an inkjet printing system that prints on a continuous web ofprint media. Printing system 100 includes a first printing module 102and a second printing module 104, each of which includes lineheads 106,dryers 108, and a quality control sensor 110. Each linehead 106typically includes multiple printheads (shown in FIGS. 2 and 3) thatapply ink or another liquid to the surface of the print media 112 thatis adjacent to the printheads. For descriptive purposes only, thelineheads 106 are labeled a first linehead 106-1, a second linehead106-2, a third linehead 106-3, and a fourth linehead 106-4. In theillustrated embodiment, each linehead 106-1, 106-2, 106-3, 106-4 appliesa different colored ink to the surface of the print media 112 that isadjacent to the lineheads. By way of example only, linehead 106-1applies cyan colored ink, linehead 106-2 magenta colored ink, linehead106-3 yellow colored ink, and linehead 106-4 black colored ink.

The first printing module 102 and the second printing module 104 alsoinclude a web tension system that serves to physically move the printmedia 112 through the printing system 100 in the transport direction 114(left to right as shown in the figure). The print media 112 enters thefirst printing module 102 from a source roll (not shown) and thelinehead(s) 106 of the first module applies ink to one side of the printmedia 112. As the print media 112 feeds into the second printing module104, a turnover module 116 is adapted to invert or turn over the printmedia 112 so that the linehead(s) 106 of the second printing module 104can apply ink to the other side of the print media 112. The print media112 then exits the second printing module 104 and is collected by aprint media receiving unit (not shown).

Although FIG. 1 depicts each printing module with four lineheads 106,three dryers 108, and one quality control sensor 110, embodiments inaccordance with the invention are not limited to this construction. Aprinting system can include any number of lineheads, any number ofdryers, and any number of quality control sensors. The printing systemcan also include a number of other components, including, but notlimited to, web cleaners and web tension sensors.

And although the printing system shown in FIG. 1 has the turnover module116 disposed in the second printing module 104, other printing systemscan include the turnover module within the first printing module 102 orbetween the printing modules.

FIG. 2 illustrates a portion of printing system 100 in more detail. Asthe print media 112 is directed through printing system 100, thelineheads 106, which typically include a plurality of printheads 200,apply ink or another liquid onto the print media 112 via the nozzlearrays 202 of the printheads 200. The printheads 200 within eachlinehead 106 are located and aligned by a support structure 204 in theillustrated embodiment. After the ink is jetted onto the print media112, the print media 112 passes beneath the one or more dryers 108which, for example, apply heat 206 to the ink on the print media orprovide a flow of air past the ink on the print media.

Referring now to FIG. 3, there is shown a side of the support structure204 that is adjacent to the print media 112 in an embodiment inaccordance with the invention. The printheads 300, 302, 304, 306, 308,310 are aligned in a staggered formation, with upstream printheads 300,302, 304 and downstream printheads 306, 308, 310, such that the nozzlearrays 312 produce overlap regions 314. The overlap regions 314 enablethe print from overlapped printheads to be stitched together without avisible seam through the use of appropriate stitching algorithms thatare known in the art. These stitching algorithms ensure that the amountof ink printed in an overlap region 314 is not higher or lower than theink on other portions of the print media.

FIG. 4 depicts a portion of a printing system in an embodiment inaccordance with the invention. Printing system 400 includes one or moreintegrated imaging systems 402 disposed over the print media 404.Although FIG. 4 illustrates only one integrated imaging systempositioned across the width (cross-track direction) of the print media,embodiments in accordance with the invention can dispose any number ofintegrated imaging systems across the width of the print media 404.

The integrated imaging systems 402 are disposed over the print media 404at locations in printing system 400 where the print media 404 istransported over rollers 406 in an embodiment in accordance with theinvention. The print media can be more stable, both in the cross-trackand in-track (media transport) directions, when moving over the rollers406. In other embodiments in accordance with the invention, one or moreintegrated imaging systems can be positioned at any location in aprinting system.

The integrated imaging systems 402 are connected to an image processingdevice 408. The image processing device 408 is adapted to process pixeldata received from the integrated imaging systems 402 and identify inkcolors and detect density variations in content printed on the printmedia 404 in an embodiment in accordance with the invention. Theintegrated imaging system or systems 402 can be connected to andtransmit data to the image processing device 408 through any known wiredor wireless connection. Image processing device 408 can be external toprinting system 400; integrated within printing system 400; orintegrated within a component in printing system 400. The imageprocessing device 408 can be implemented with one or more processingdevices, such as a computer or a programmable logic circuit.

Motion encoder 410 can be used to produce an electronic pulse or signalproportional to a fixed amount of incremental motion of the print mediain the feed direction. The signal from motion encoder 410 is used totrigger an image sensor (see 506 in FIG. 5) to begin capturing an imageof the printed content on the moving print media using the lightreflected off the print media.

Connected to the image processing device 408 is memory storage device412. The storage device 412 can store reference density values, forexample, included in a series of look up tables (LUTs), and pixel datavalues used to identify density values and ink colors in an embodimentin accordance with the invention. The storage device 412 can beimplemented as one or more external storage devices; one or more storagedevices included within the image processing device 408; or acombination thereof.

FIG. 5 is a cross-sectional view along line 5-5 in FIG. 4 in anembodiment in accordance with the invention. Integrated imaging system402 includes light source 500, transparent cover 502, folded opticalassembly 504, and image sensor 506 all enclosed within housing 510. Inthe illustrated embodiment, folded optical assembly 504 includes mirrors512, 514 and lens 516. Mirrors 512, 514 can be implemented with any typeof optical elements that reflects light in embodiments in accordancewith the invention.

Light source 500 transmits light through transparent cover 502 andtowards the surface of the print media (not shown). The light source cancomprise a broad spectrum light source such as an incandescent light orfluorescent light, or can comprise light sources that emit light in oneor more narrow bands such as LEDs, lasers or gas discharge lightsources. If the light source comprises light sources having a narrowwavelength emission spectrum, multiple narrow band light sources can beused, having different narrow wavelength emission spectra to coverdifferent portions of the spectra. For example the light source 500 maycomprise a set of different color LEDs. Although not shown in FIG. 6,the light source can be extended in length to span the width of theprint media so that uniform illumination is provided across the width ofthe print media. The light reflects off the surface of the print mediaand propagates through the transparent cover 502 and along the foldedoptical assembly 504, where mirror 512 directs the light towards mirror514, and mirror 514 directs the light toward lens 516. The light isfocused by lens 516 to form an image on image sensor 506. Image sensor506 captures one or more images of the print media as the print mediamoves through the printing system by converting the reflected light intoelectrical signals.

Folded optical assembly 504 bends or directs the light as it istransmitted to image sensor 506 such that the optical path traveled bythe light is longer than the size of integrated imaging system 402.Folded optical assembly 504 allows the imaging system 402 to beconstructed more compactly, reducing the weight, dimensions, and cost ofthe imaging system. Folded optical assembly 504 can be constructeddifferently in other embodiments in accordance with the invention.Additional or different optical elements can be included in foldedoptical assembly 504.

As discussed earlier, image sensor 506 can receive a signal from amotion encoder (e.g., 410 in FIG. 4) each time an incremental motion ofthe print media occurs in the feed direction. The signal from the motionencoder is used to trigger image sensor 506 to begin integrating thelight reflected from the print media. In the case of a linear imagesensor, the unit of incremental motion is typically configured such thatan integration period begins with sufficient frequency to sample orimage the print media in the feed direction with the same resolution asis produced in the cross-track direction. If the trigger occurs at arate which produces a rate that results in sampling in the in-track(feed) direction at a higher rate, an image that is over sampled in thatdirection is produced and the imaged content appears elongated orstretched in the in-track direction. Conversely, a rate that is lowerfor the in-track direction produces imaged content that is compressed inthe in-track direction.

The time period over which the integration occurs determines how muchprint media moves through the field of view of the imaging system. Withshorter integration periods such as a millisecond or less, the motion ofthe print media can be minimized so that fine details in the in-trackdirection can be imaged. When longer integration periods are used, thelight reflected off the print media is collected while the print mediais moving and the motion of the print media means the printed content isblurred in the direction of motion. The blurring in the direction ofmotion has the effect of averaging the pixel data in one direction, thein-track (feed) direction. Averaging the pixel data through blurring isalso known as optical averaging. By performing the averaging opticallywith longer integration periods, the amount of data that is transferredto and processed by a processing device (e.g., 408 in FIG. 4) isreduced. Blurring reduces image resolution in the in-track direction,and is therefore generally avoided for applications that require theidentification of artifacts that are small and occur randomly.

The amount of optical averaging can be increased by reducing thefrequency of the pulses from the motion encoder (e.g., 410 in FIG. 4)and extending the integration time of the image sensor (e.g., 506 inFIG. 5) in the imaging system (e.g., 402 in FIG. 5). Reducing thefrequency of the pulses has the benefit of reducing the amount of datatransferred to the image processing device and of reducing the numericalaveraging performed by the image processing device (e.g., 408 in FIG.4). The integration time should be less than the period between imagecapture events. Additional numerical averaging or other image processingof the pixel data in the in-track direction can be computed by theprocessing device on images captured by the image sensor. The amount ofoptical image averaging can be decreased with an increase in thenumerical averaging required. The ability to use optical averaging notonly significantly reduces the camera hardware cost, but also itsfootprint size.

In another embodiment in accordance with the invention, averaging of thepixel data in one direction can be performed by a processing device(e.g., 408 in FIG. 4) using multiple images captured by the imagesensor. The images can be captured with shorter integration times in anembodiment in accordance with the invention. The processing devicenumerically averages the pixel data in one direction, the in-trackdirection, to produce blurring in an image or images. The processingdevice can also perform other types of imaging processing procedures inaddition to the numerical averaging of the pixel data.

Returning to FIG. 5, the transparent cover 502 is disposed over anopening 501 in the housing 510. Transparent cover 502 is optional andcan be omitted in other embodiments in accordance with the invention.Integrated imaging system 402 can also include vent openings 518, 520.Vent opening 518 can be used to input air or gas while vent opening 520can be used to output exhaust. The input air or gas can be used tomaintain a clean environment and control the temperature withinintegrated imaging system 402. In another embodiment in accordance withthe invention, integrated imaging system 402 can include one or morevent openings (e.g., vent opening 518) that input air or gas and theopening 501 in the housing 510 is used to output exhaust. The outputopening can be positioned such that it directs clean dry air across theexterior face of the transparent cover 502 to ensure that the exteriorface of the transparent cover 502 remains clean and dry. In embodimentin which the opening 501 doesn't include a transparent cover 502, theinput gas or air can flow out through the opening 501 to prevent theflow of moisture of dirt into the housing where they can contaminate theoptical components.

FIG. 6 is a cross-sectional view along line 6-6 in FIG. 4 in anembodiment in accordance with the invention. As described, light source500 transmits light through transparent cover 502 and towards thesurface of the print media (not shown). The light reflects off thesurface of the print media, propagates along folded optical assembly,and is directed toward lens 516. Lens 516 focuses the light to form animage on image sensor 506. Light source 500 can remain on or can bestrobed at a rate appropriate for the integration time of the imagesensor 506. Image sensor 506 can be implemented with any type of imagesensor, including, but not limited to, one or more linear image sensorsconstructed as a charge-coupled device (CCD) image sensor or acomplementary metal oxide semiconductor (CMOS) image sensor. The imagesof the print media formed on the image sensor 506 are converted to adigital representation that is suitable for analysis in a computer orimage processing device, such as image processing device 408.

Referring now to FIG. 7, there is shown a flowchart of a method forcolor density correction in a printing system in an embodiment inaccordance with the invention. As described earlier, variations in inklay down characteristics between printheads can lead to unpredictablevariations in dark and light density regions. The method of FIG. 7 isdescribed in conjunction with one printhead in a linehead, but thoseskilled in the art will recognize the method can be used continuously orat select times with one or more printheads in one or more lineheads.

Initially, a printhead in a linehead prints a test block having a knownor fixed print density on a print media (block 700). The test block caninclude any given content having a known print density. The test blockcan be included in a test block pattern in an embodiment in accordancewith the invention. FIG. 8 illustrates one example of a test blockpattern 800. The test block pattern 800 includes multiple test blocks802, 804, 806, 808, 810, 812. In the illustrated embodiment, each testblock has a known density that is different from the density of theother test blocks in the test block pattern. By way of example only,test block 802 can have a density of 0.2, test block 804 a density of0.4, test block 806 a density of 0.6, test block 808 a density of 0.8,test block 810 a density of 1.0, and test block 812 a density of 1.2.

Other embodiments in accordance with the invention can include anynumber of test blocks in a test block pattern. If a test block patternhas two or more test blocks, at least two of the test blocks can havediffering known densities.

Returning to block 702 in FIG. 7, the printed test block is scanned andthe pixel data averaged in the in-track direction to produce pixel datavalues for the printed test block and a measured density value trace forthe printhead. As used herein, the term trace can be a graph of themeasured density value data points or a non-graphed array of themeasured density value data points. In some embodiments, the storagedevice 412 stores measured density value data, for example, in a seriesof look up tables (LUTs). The pixel data is optically averaged in theillustrated embodiment. The pixel data can be numerically averaged inanother embodiment in accordance with the invention.

The color of the ink or substance that was printed on the print media isthen identified at block 704 using the pixel data values obtained fromscanning the printed text block. In one embodiment in accordance withthe invention, three linear image sensors are used to scan the testblock. The image sensors have different color filter arrays disposedover the photosensitive sites. A color array includes color filterelements that each transmits light propagating within a known wavelengthrange. The color filter elements block or absorb light propagatingoutside the known wavelength range. Thus, the photosensitive sites inthe linear image sensor detect light propagating within the knownwavelength range. The wavelength sensitivities of the color filterarrays are selected to be complementary colors to the colors in the inkin an embodiment in accordance with the invention.

For example, in a printing system that uses cyan, magenta, and yellowcolored inks, one linear image sensor can include a red color filterarray, one linear image sensor a blue color filter array, and the thirdimage sensor a green color filter array. The photosensitive sites in thelinear image sensor with the red color filter array detect lightpropagating within the wavelength range associated with the color red.The photosensitive sites in the linear image sensor with the blue colorfilter array detect light propagating within the wavelength rangeassociated with the color blue. And the photosensitive sites in thelinear image sensor with the green color filter array detect lightpropagating within the wavelength range associated with the color green.

The linear image sensors each produce pixel data values representing theamount of light detected by the photosensitive sites. Thus, in theexample embodiment that uses three image sensors for the cyan, magenta,and yellow colored inks, three pixel data values are produced for eachtest block in an embodiment in accordance with the invention. Otherembodiments in accordance can include a different number of imagesensors or a different number of ink colors.

The three pixel data values are used to determine the color of theprinted test block. FIGS. 9-11 illustrate one method for identifying acolor. FIG. 9 depicts an example of a spectral response plot in anembodiment in accordance with the invention. In FIGS. 10 and 11, plot900 represents the spectral response capture cross section of the imagesensor having a blue color filter array, plot 902 the spectral responsecapture cross section region of the image sensor having a green imagesensor, and plot 904 the spectral response capture cross section regionof the image sensor having a red color filter array. The absorbancespectral response values are plotted for a yellow ink, a magenta ink, acyan ink, and a black ink. The intersection of the capture cross sectionplots and the colored ink absorbance plots are used to identify thecolor and color density of the scanned test blocks. The capture crosssections illustrated in FIGS. 9-11 represent the capture response of theimage sensors (e.g., the total amount of light the image sensors cansense).

For example, in FIGS. 9 and 10, the area under the magenta inkabsorbance spectral response that overlaps with the capture crosssection of the image sensor with the green color filter array (plot 902)is a measure of the amount of light captured by this sensor from ascanned color block. The overlapped area is shown as the hashed area1000 in FIG. 10. In this example, the capture value for the magenta inkthat is represented by the hashed area 1000 is output from the imagesensor having the green color filter array.

A smaller overlap area (hashed area 1002) between the magenta inkabsorbance response and the capture cross section (plot 900) of theimage sensor with the blue color filter array is shown in FIG. 10. Inthis example, the capture value for the magenta ink output from theimage sensor with the blue color filter array would be relatively low.

Finally, there is no overlap between the magenta ink absorbance spectralresponse with the capture cross section of the image sensor having thered color filter array (plot 904). Hence the capture value for themagenta ink from the image sensor with the red color filter array issubstantially zero. The ratio of the three capture values for a testblock having a given ink color can be used to identify the color. Theabsolute capture values output from the three image sensors for a testblock can be used to determine the density of the color block. This isone example of a technique for using only three image sensors in animaging device to determine both the color and the color density of afixed color block.

Similarly, the area under the yellow ink absorbance spectral responsethat overlaps with the capture cross section of the green color filterarray sensor (plot 902) is a measure of the amount of light captured bythis sensor from a scanned color block. The overlapped area is shown asthe hashed area 1100 in FIG. 11. In this example, the capture value forthe yellow ink that is represented by the hashed area 1100 is outputfrom the image sensor having the green color filter array.

A larger overlap area (hashed area 1102) between the yellow inkabsorbance response with the capture cross section (900) of the imagesensor with the blue color filter array is shown in FIG. 11. In thisexample, the capture value for the magenta ink output from the imagesensor with the blue color filter array would be relatively higher thanthe capture value produced by the image sensor with the green colorfilter array. Finally, there is no overlap between the magenta inkresponse with the red sensor (904). Hence the captured value for themagenta ink from the image sensor with the red color filter array issubstantially zero. The ratio of these the capture values for a testblock having a given ink color can be used to identify the color. Theabsolute capture values output from the three image sensors for a testblock can be used to determine the density of the color block. This isone example of a technique for using only three image sensors in animaging device to determine both the color and the color density of afixed color block.

If the individual test blocks of the test pattern 800 are each printedwith a single ink, the analysis of the ratio of these the capture valuesfor a test block having a given ink color can be used to confirm thatthe printed ink has a similar absorption spectra to the ink intended forprinting the test block. This can be used to confirm whether the printedink is approved for use in the printer or whether it may be anon-approved ink that could adversely affect the operation of theprintheads. Should a non-approved ink be detected, the printing systemmay notify the operator that non-approved ink may invalidate thewarranty of the printheads or fluid system.

Typically, the absorbance spectral response for a test block increaseswhen the density of the test block increases. An increase in the colordensity produces an overlap between the absorbance spectral response andthe capture cross section of an image sensor that is greater. Theincrease in the absorbance spectral response is shown with spectralresponses 1004, 1006, and 1008 in FIG. 10.

The absorbance spectral response for a test block decreases when thedensity of the test block decreases. An increase in the color densityproduces an overlap between the absorbance spectral response and thecapture cross section of an image sensor that is smaller. The decreasein the absorbance spectral response is shown with spectral responses1010 and 1012 in FIG. 10. Thus, changes in the absorbance spectralresponses correspond to changes in the density of the test blocks.

Returning to block 706 in FIG. 7, the density of the scanned test blockis determined using the pixel data values obtained from scanning thetest block. The absorption values for each ink color depend on the colordensity of the test block. As the color density of the test blockincreases, the absorption values increase. And as the color density ofthe test block decreases, the absorption values decrease.

Next, as shown in block 708, the measured density value trace iscompared with a reference density value trace. By way of example only,reference density values can be independently supplied by a printingsystem manufacturer or customized or set by the user of the printingsystem. FIG. 12 illustrates one example of density value traces for theprintheads shown in FIG. 3 in an embodiment in accordance with theinvention. Trace 300′ corresponds to printhead 300, trace 302′ toprinthead 302, trace 304′ to printhead 304, trace 306′ to printhead 306,trace 308′ to printhead 308, and trace 310′ to printhead 310. Thereference density value is represented by plot 1200.

A determination is made at block 710 of FIG. 7 as to whether or not adifference between the measured density value trace and the referencedensity value equals or exceeds a threshold value. If the differenceequals or exceeds the threshold value, the ink laydown for the printheadis adjusted at block 712 based on the difference between the measureddensity value trace and the reference density value. For example, asshown in FIG. 10, the measured density value trace 308′ is greater thanthe reference density value 1200. If the difference equals or exceeds athreshold value, the ink laydown for printhead 308 is adjusted based onthe difference. Examples of techniques that can be used to adjust theink laydown include, but are not limited to, changing the size of theink drops jetted by the printhead, by changing the ink pressure, or byaltering the halftoning algorithm to change the number of ink dropsjetted by the printhead. One change or a combination of changes can beimplemented to adjust the ink laydown. The change (or changes) producesa printed density that is the same, or substantially the same as thereference density value.

Embodiments in accordance with the invention can perform the methodshown in FIG. 7 one or more times. For example, the method of FIG. 7 canbe performed each day prior to beginning any print jobs to calibrate theprinting system; or the method of FIG. 7 can be performed during a printjob to monitor and correct for any flat field errors that develop duringthe print job.

Embodiments in accordance with the invention can perform the methodshown in FIG. 7 differently or can include additional functions orprocesses. Additionally, some of the blocks can be omitted in otherembodiments in accordance with the invention. By way of example only,block 710 can be omitted.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention. And even though specific embodiments of the inventionhave been described herein, it should be noted that the application isnot limited to these embodiments. In particular, any features describedwith respect to one embodiment may also be used in other embodiments,where compatible. And the features of the different embodiments can beexchanged, where compatible.

Parts List

-   100 printing system-   102 printing module-   104 printing module-   106 linehead-   108 dryer-   110 quality control sensor-   112 print media-   114 transport direction-   116 turnover module-   200 printhead-   202 nozzle array-   204 support structure-   206 heat or air-   300 printhead-   300′ measured density value trace for printhead 300-   302 printhead-   302′ measured density value trace for printhead 300-   304 printhead-   304′ measured density value trace for printhead 300-   306 printhead-   306′ measured density value trace for printhead 300-   308 printhead-   308′ measured density value trace for printhead 300-   310 printhead-   310′ measured density value trace for printhead 300-   314 overlap region-   400 printing system-   402 integrated imaging system-   404 print media-   406 roller-   408 image processing device-   410 motion encoder-   412 storage device-   500 light source-   501 opening in housing-   502 transparent cover-   504 folded optical assembly-   506 image sensor-   510 housing-   512 mirror-   514 mirror-   516 lens-   518 vent-   520 vent-   800 test block pattern-   802 test block-   804 test block-   806 test block-   808 test block-   810 test block-   812 test block-   900 plot of spectral response region-   902 plot of spectral response region-   904 plot of spectral response region-   1000 overlap area-   1002 overlap area-   1004 absorbance spectral response of test block with increased    density-   1006 absorbance spectral response of test block with increased    density-   1008 absorbance spectral response of test block with increased    density-   1010 absorbance spectral response of test block with decreased    density-   1012 absorbance spectral response of test block with decreased    density-   1100 overlap area-   1102 overlap area-   1200 reference density value

The invention claimed is:
 1. A method for color density correction in aprinting system that includes a linehead that jets ink onto a movingprint media and an integrated imaging system that captures images ofcontent printed on the moving print media, wherein the linehead includesone or more printheads, the method comprising: producing one or morepixel data values and a measured density value trace for a printed testblock by scanning the test block and averaging pixel data in a printmedia transport direction; determining a color and a density of the inkin the printed test block using the pixel data values; comparing themeasured density value trace with a respective reference density value;determining whether there is a difference between the measured densityvalue trace and a reference density value; and if there is a difference,adjusting ink laydown for the printhead based on the difference.
 2. Themethod as in claim 1, further comprising a printhead printing a testblock having a known density on the print media.
 3. The method as inclaim 1, further comprising determining whether the difference betweenthe measured density value trace and the reference density value equalor exceed a threshold value prior to adjusting ink laydown for theprinthead based on the difference.
 4. The method as in claim 1, whereindetermining a color and a density of the ink in the printed test blockusing the pixel data values comprises determining an overlap areabetween an absorbance spectral response of the test block with a captureresponse of an image sensor.